The consolidation of the Web as one of the most important ways to share and spread information has given raise to a huge amount of information systems for this media. This is the reason why a lot of different modelling techniques and methodologies for Web Information Systems (WISs) development have appeared. MIDAS, a model driven methodology for the development of WISs, is the framework of this paper and proposes to use UML as a unique notation to model the whole system. Despite UML does not support each of the techniques that are necessary for the development of WISs, it has some extension mechanisms that allow extending this language for other needs. In this paper a UML extension to represent XML Schemas is described.